On an OFDM system, symbols are transmitted using a plurality of simultaneous windowed sinusoidal sources operating over a series of regular time intervals, typically 3.2 us in duration, with a gap between symbols of 0.8 us to allow for separation between individual OFDM symbols. Transmission of data using OFDM modulation provides intrinisic resistance to multipath reflection at the receiver by virtue of using a plurality of subcarriers within each symbol such that each symbol carries a large amount of information in the form of a large plurality of subcarrier pilots, and each symbol has a temporal duration longer than multi-path reflection. In this manner, multi-path reflections along the communications channel and superimposed at the receiver tend to occur within the duration of a symbol time, and the symbols are separated by a guard band to ensure that bleeding from one symbol into another is minimized. One of the requirements for proper demodulation of OFDM symbols is detection of the packet and synchronization of the local receiver to the stream of symbols, as offsets between the transmitted frequency and local oscillator will result in phase ramps of the demodulated pilots. Packet detection and symbol synchronization are achieved using a short preamble part of the packet comprising a series of short symbols. Coarse adjustment of the local oscillator is achieved during a first preamble time, and fine adjustment is accomplished during a second long preamble time.
FIG. 1 shows a typical IEEE 802.11a or 802.11g OFDM packet 10 and includes a short preamble section 12 comprising ten identical short symbols shown as t1-t10. One of the short symbol t1 14 is expanded to show that it comprises a pattern of 16 OFDM samples shown as s0 through s15. The first preamble section 12 is used to achieve receiver synchronization whereby the receiver detects the short preamble and initially synchronizes to the repeating pattern t1-t10 of short preamble, each of which is 0.8 us in duration and comprises 16 OFDM samples s0 through s15, as shown for exemplar short symbol t1 14. A packet start output is asserted when the second short symbol is received, and a symbol timing output is asserted upon the passing of the final short symbol t10 of short preamble 12, as will be explained later. The packet detection signal is used to gate the operation of the symbol timing function, and when the symbol timing output is asserted, this signal is used to establish the symbol timing of all subsequent symbols in the packet, including the placement of guard bands to exclude certain areas between OFDM symbols in the data area of the packet. During short preamble section 12, an estimate of coarse frequency offset is made and fed back to the Numerically controller oscillator (NCO) for compensating frequency offset. In this manner, when the symbol timing output is asserted, the coarse frequency correction is completed, thereby allowing the symbol timing output to be more accurate over the rest of the packet. The coarse frequency correction derived from the short preamble is fed back as a first correction to tune the NCO, and during a Long preamble interval, a longer and more accurate estimate of frequency offset is made and fed back to the NCO, as is known to one skilled in the art of wireless receivers. The short preamble section 12 comprising a plurality of short symbols t1 through t10 is followed by a guard band 18 to separate the short preamble and long preamble section, and this is followed by long preamble section 20, which comprises two long symbols, shown as T1 22 and T2 24. The long symbols T1 22 and T2 24 are used to establish a more precise estimate of frequency offset after the initial oscillator adjustment is made based on the short preamble coarse frequency offset measurement.
The functions of packet detection, symbol timing, and coarse frequency offset have been performed by separate or combined circuit functions in the prior art.
U.S. Pat. No. 6,320,915 by Stott et al describes a symbol synchronization system whereby an incoming stream of symbols is delayed and multiplied by the complex conjugate of the incoming stream of symbols, thereby generating a synchronization pulse for further processing. U.S. Pat. No. 6,539,063 by Peyla et al describes a system for recovering symbol timing and carrier frequency error using the product of multiplication of an OFDM symbol stream with a slightly delayed version, thereby extracting symbol synchronization and timing from the energy introduced by shaping the leading and trailing edges of the subcarriers used in OFDM. U.S. Pat. No. 6,658,063 by Mizoguchi et al. describes a symbol timing recovery mechanism for 802.11 OFDM WANs, and includes a coarse frequency offset. U.S. Pat. No. 6,675,012 by Gray describes the use of multiplying a signal with a conjugated version of the signal which has been delayed by one symbol time, thereby providing a means for packet detection.
U.S. Pat. No. 6,363,084 by Dejohnge describes a method for extracting coarse frequency offset by performing an FFT on the same sample point of multiple preamble symbols to detect the offset frequency. U.S. Pat. No. 6,381,263 by Suzuki describes a system for measuring frequency offset in a multi-channel system, and U.S. Pat. No. 6,459,679 by Kim describes a system for extracting frequency offsets in a single channel OFDM system.
FIG. 2 shows a prior art packet detection system 60 and a prior art frequency offset measurement system 40, while FIG. 3 shows a prior art symbol timing generator 100. These may be best understood in combination with the waveforms of FIG. 4. Packet detection is performed in FIG. 2 by examining a stream of complex OFDM symbols 92, as would be generated by the baseband detection of a modulated stream of OFDM symbols. One of the requirements of the demodulation system is to determine the start of the packet, which is asserted by packet detection signal 90. The incoming complex OFDM symbols 92 are delayed by one short preamble symbol interval L (shown for the case L=16) by delay element 62. A first multiplier 66 multiplies each incoming symbol from the incoming stream 92 (shown as stream 120 in FIG. 4) with a delayed and conjugated stream from conjugator 64. The modulation of OFDM symbols uses a cosine weighting function, which produces a real and imaginary component which is found in an OFDM symbol stream, and first multiplier 66 produces an output stream which is complex and has a large magnitude component, as shown in waveform 122 of FIG. 4. A sum of the previous sixteen symbols forming the correlation with the present symbol and previous symbols is performed in first summer 70, which produces the waveform 126 of FIG. 4, and the magnitude of this value is determined in absolute value 80, thereby producing a value 84 which is the absolute value of Cn, which is related to a running correlation of a current symbol with a previous symbol. When noise is entering the system, there is little or no correlation between these two intervals, and the output 74 has only noise level components. At the time the second preamble is received, the correlation is very high between the first preamble and second, as shown in waveform 122 of FIG. 4, and the first summer 70 accumulates an increasing value, which has a maximum value at the terminus of the second preamble t2 of FIG. 1, since the value is maximized at this point, as seen in waveform 126 of FIG. 4. The second multiplier 68 of FIG. 2 is simply finding the power of the previous symbol by multiplying the delay 62 output with a conjugate value generated by conjugator 64, which has the value of the signal plus noise power. The output of second multiplier 68 as shown by waveform 124 in FIG. 4 and represents the magnitude of the incoming signal regardless of correlation to previous symbols, while the output of the first multiplier 66 represents the magnitude of incoming signal which is correlated one short preamble time (shown for preamble length L=16) earlier. The first multiplier 66 output (waveform 122 of FIG. 4) tracks correlation and signal level, and the second multiplier output 68 (waveform 124 of FIG. 4) tracks signal level only. By finding the absolute value, or magnitude, of the output of the first summer 70 and generating Cn 84 representing the correlated power, and dividing by Pn 76, we may generate a packet detection function:u=|Cn|/(Pn)
and when u is above a threshold level 132 as shown in waveform 130 of FIG. 4 as determined by comparator 88, we may assert packet detection signal 90, shown as waveform 134 of FIG. 4.
The coarse frequency offset estimation is performed by block 40 of FIG. 2. The incoming symbol stream is delayed and conjugated and multiplied together by multiplier 44 to generate an output which contains frequency offset equivalent constant phase offset, which is provided as an_input to a phase finder 46, which is a CORDIC (COordination Rotation DIgital Computer) function as is known to one skilled in the art. Phase finder 46 takes a complex-valued input and generates an output which is a phase value. Averaging the output of this phase finder generates an output which is the coarse frequency offset 50, however it can be shown that averaging 48 this output does not improve the noise rejection of the coarse frequency offset 50 in poor s/n conditions.
FIG. 3 shows a prior art symbol timing generator 100. Input complex OFDM symbol stream 92 as was applied to FIG. 2, is input to a tapped delay element 102 having a number of taps equal to the short preamble length (shown for preamble length L=16), which performs a cross correlation of incoming complex OFDM symbol stream 92 to a short preamble symbol 104 using multiplier 106 to multiply the moving OFDM stream against the fixed preamble symbols 104. The cross correlation contributions of each multiplication are summed in adder 108, which produces a cross correlation output stream 111 which is converted to a magnitude in absolute value function 113, which produces the output 115 shown as waveform 136 of FIG. 4, where each correlation between the incoming symbol stream and short symbol produce a correlation peak, as shown. This cross-correlation output stream 115 is fed to a second tapped delay element 110, where each delay is equal to the length of an entire short symbol (shown for L=16), such that the correlation peaks for all 10 short symbols are summed in adder 112. This results in the adder 112 output 118 increasing, shown as waveform 138 of FIG. 4, where each additional correlation peak causes the adder 112 output 118 to increase to a maximum at the final correlation peak, as shown. After the final correlation peak, the output 118 of the adder 112 begins to decrease, and this is detected by max peak finder 114, which generates the output 116 symbol timing pulse shown in waveform 139 of FIG. 4. In this manner, a prior art symbol timing output is generated.
There are several shortcomings of the prior art circuitry. While these system perform well in high signal to noise conditions, the operation in low signal to noise conditions is compromised. As the noise level of waveforms 122 and 124 of FIG. 4 increases, the values of Cn and Pn 126 and 128 become increasingly noisy, and when generate a function u which has a high level or rms noise, thereby causing the start of packet signal to become unreliable. Further, the cross-correlation output stream 117 of FIG. 3 is also effected by this noise, which results in generation of spurious symbol timing output 116.
It is desired to integrate the functions of symbol timing, packet detection, and coarse frequency offset measurement. It is also desired to provide an improved symbol timing signal which operates reliably in the presence of high levels of noise compared to prior art symbol timing and start of packet detectors.